Recently, the rapid development of smaller, lighter, and higher performance electronic and communication equipment has required the development of high performance and large capacity batteries to power such equipment. The demands for large capacity batteries have prompted investigation into rechargeable lithium batteries. Positive active materials for rechargeable lithium batteries use lithium-transition metal oxides, and negative active materials use crystalline or amorphous carbonaceous materials or carbon composites. The active materials are coated on a current collector with a predetermined thickness and length, or they are formed as a film, to produce electrodes. The electrodes together with a separator are wound to produce an electrode element, and the electrode element is inserted into a battery case such as a can followed by insertion of an electrolyte to fabricate a battery.
The electrolyte includes lithium salts and organic solvents. The organic solvents may be mixed solvents of between two and five components which may include cyclic carbonates such as ethylene carbonate and propylene carbonate, and linear carbonates such as dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate. However, these solvents are known to severely expand at high temperatures, causing a swelling phenomenon. The swelling phenomenon is due to the generation of gas due to the decomposition of the electrolyte at high temperatures in the battery.
Several attempts to reduce such a swelling phenomenon have been attempted. U.S. Pat. No. 4,830,939 discloses a polymer electrolyte including polyethylenically unsaturated monomeric materials or prepolymonomeric materials, and U.S. Pat. No. 4,886,716 discloses a polymer electrolyte including a cross-linked polyether which is a copolymerization product of a vinyl-ether. U.S. Pat. No. 4,908,283 discloses a polymer electrolyte including acryloyl-denatured polyalkylene oxide.
The conventional attempts for reducing the high-temperature swelling phenomenon are based on the cross-linking of a multifunctional monomer in which the main chain is a poly(alkylene oxide) or a polyalkylene unit. However, these attempts still have problems associated with the high-temperature swelling phenomenon.